The present invention relates to a method for sawing a tree trunk, in which method, along with the sawing process, there are defined the trunk dimensions and shape, as well as the biological quality of each section of the trunk, both in the longitudinal and the transversal direction, and wherein these characteristics are taken into account while treating the trunk.
The invention also relates to a method for treating a uniformly thick slice of wood sawed off the trunk, wherein the shape of the slice of wood and its quality are defined, whereafter the slice is cut and sawed according to the said definitions.
With respect to quality, a tree trunk is divided into three sections in the lengthwise direction. The butt end is a knot-free section, because the branches have pruned naturally at an early stage, and consequently the butt end is valuable wood material as a whole. The following section contains dead knots, part of which remain inside the trunk and part of which extend to the trunk surface. The third section is full of sound knots up to the top end of the trunk. When a log is sawed into lumber, it is important that the individual articles are of a homogeneous quality, i.e. that they are sawed off the same quality section. The value, or the quality, of sawed goods is determined with respect to the quality defects contained therein. If the beginning of a sawed product is knot-free but the rest contains dead, rotten knots, the quality of the whole article is determined according to the latter. If the length of the knot-free section is over 1.8 m, the knotty part can be cut off. The quality is improved, but the quantity is decreased.
The term `sawed goods` generally refers to such wood material which is produced by means of sawing or in a similar manner of tree trunks, i.e. logs, and processed into articles with certain standard dimensions (thickness.times.width.times.length) which are pre-dried and sorted out according to certain quality grades (for instance unsorted, fifth-grade and sixth-grade).
According to the use of the final product, sawed goods of different qualities often have different dimensions. In order to improve the process of sawed goods, logs containing different wood materials should therefore be sawed into several different thicknesses and widths, and in some cases also into different lengths. While in the currently used method of sawing the thickness of sawed goods, as well as the width of the center-part product are defined already when the blade settings are made, i.e. beforehand, the practice is to try and assess the quality of the logs already at the cutting site in the wood, as well as at the saw mill in connection to the sorting of the logs. The quality assessment is carried out on the basis of the visible knot border on the outer surface of the log, or on the basis of knot bumps. Part of the knots, however, are inside the trunk, and therefore it is impossible to accurately define any exact quality sections. In order to find out the quality borders, for instance X-ray devices have been tried, but they have not fulfilled the expectations, and they cannot be used at the cutting site in the wood.
Better information of the quality is received in the so-called double cutting, where in the first breakdown the two opposite sides of a log are sawed into boards, whereafter in the second breakdown the center part, i.e. the cant, is turned on its flat side and sawed into several pieces either by means of a frame saw, a multiple-blade circular saw or by several band saws. The sawn faces of the cant tell a lot more about the quality of the log than the round log as such. Logs of different qualities can now be sawn with different blade settings, if the saw mill has been planned so that various different thicknesses can be sawn either by means of parallel sawing machines or such equipment where the blade settings can be quickly adjusted. This arrangement, however, increases the expenses because of extra machinery, and causes problems with accuracy when several blades must be set simultaneously. The time needed for setting lowers the capacity, because the intervals between logs are lengthened. In both methods, the number of possible setting alternatives is fairly limited. Moreover, the sawing thickness of the first side cuts must be estimated on the basis of the log surface. In the so-called travelling carriage sawing method, a log can be sliced into desired thicknesses starting from the outer surface and proceeding towards the heart, so that the first-sawed face gives a hint of the quality of the following piece, and the thickness of the following piece can still be determined at this stage. But in this method, too, the deliberately or naturally pruned knot section comes as a surpise.
Among the greatest drawbacks in the current methods of sawing tree trunks is the fact that the trunks are cut into logs before the actual sawing stage. Although an erroneous cutting were registered as the sawing proceeds, the damage is already done and cannot be corrected; the only possibility is to try and minimize the created damage by special operations. In some sawing methods it is possible to crosscut a piece sawn off the log, for instance a side cut component, and to keep the resulting pieces for further processing. The already performed stages of treatment, such as cutting into logs and sawing, limit the alternative choices in further processing. In the case of the above example, the thickness and short total length create a limit, in which case the achieved benefit remains small and often unprofitable. Therefore this alternative has in practice disappeared from the sawmill industry.
Ordinary lumber is delivered from a sawmill for further processing, for instance in furniture industry. In industrial processing, the sawed goods are treated in the following ways: they are dried, split, trimmed and sorted, and the received product components are grouped into suitable component bundles according to the specific needs of the production, whereafter the final products or their various parts are manufactured of the said product components and finished for example by means of planing, grinding and surface finishing.
When industrial processing makes sawed goods into various wood products, the resulting proportion of wasted raw material rises high. The amount of trimming and quality waste may rise up to 15-30 %, sometimes even higher. Naturally this is a considerable factor in the total expenses. At the same time it is a factor of great uncertainty in the production, because the final amount of faltless material received from the purchased sawed goods may vary remarkably.
The reason for the creation of trimming waste is that the length of the sawn goods is unsuitable with respect to the length of the final product. Generally sawed goods are sold in lengths as-sawed, in which case some lengths may be suitable and others remarkably oversized, resulting in lengthy rejects. Another reason for trimming waste is that the goods contain defects which are not acceptable in the final product. Consequently they must be removed at the trimming stage. These defects cause cutting waste also when it is possible to buy cut-to-size sawed goods which are exactly suitable as regards the length of the final product. In addition to the waste created while trimming off defects, other reject pieces are created at the same time.
Apart from the elimination of single defects, quality waste is also created because part of the sawn goods does not correspond to the quality required by the final product in question. In practice sawed goods are graded according to the general grading standards for export lumber, i.e. into unsorted, fifth-grade, sixth-grade and schaalboard, and this classification rarely matches the quality requirements set for the final product. Therefore whole articles of sawed goods must sometimes be rejected, and while trimming, part of the article must be removed for secondary use.
The amount of created waste can be decreased if the manufacturer can use raw material with the same original dimensions but with several different qualities and lengths. The reject pieces can also be joined or glued together, if there is demand for such secondary sawed goods. This, however, leads to extra work and expense. Generally different qualities also have different dimensional requirements.
In the most difficult situation are such further processors of sawed goods need only one specific length of a raw material with a homogeneous quality. Finnished sawed goods can only be processed into such raw material with a high proportion of waste. Therefore many manufacturers have started to use African wood species which are uniform in quality--irrespective of their high price.